Critics of human folly such as Dewey (1933) have suggested that we tend to be impulsive, to cut off our thinking prematurely, and thus to reach conclusions we regret. Others (e.g., Simon, 1957) have argued that thinking is often not worth the effort. To discover whether, and when, we are impulsive in reaching conclusions, we must compare our thinking to a normative model that specifies when we ought to stop. I present a model (Savage, 1954), which defines the value of further thinking - in this case, further gathering of evidence - as the expected gain from further thinking minus the cost. Behavior of subjects is compared to the model in a variety of laboratory tasks. In many of these tasks, explicit payoffs are given for good performance, and explicit costs are imposed for time. Subjects must decide how long to work on a problem in order to maximize their payoff. These experiments also seek a relationship between overconfidence and impulsiveness. In some of these, subjects are provided with feedback that they might be wrong about an answer, and they then reevaluate their confidence; rigidity, or lack of responsiveness to this suggestion, might itself cause impulsiveness. In other experiments, subjects ask questions in order to discover a rule. These experiments address specific biases that might lead to premature cessation of thinking, such as a tendency to ask questions whose answer will be "yes", and a tendency to consider the relevance of the question to only one's most favored hypothesis. Other experiments seek such biases directly by asking subjects to assess the value of questions when relevant probabilities are given. In all of these experiments, individual differences will be examined, and training will be attempted, if any biases are found. The experiments should shed light on the needs for education in good thinking, hence, on a possible aspect of teachable intelligence. Biases of the sort examined have also been implicated in various forms of psychopathology.